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Color Fundamentals: Advanced Coloring

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What are materials and textures? How can we see transparent things? What makes a scene something more than a set of objects? How do you mix reflected colors? Learn tricks that will bring life to your realistic paintings!

This is the the last part of our mini series on color and light. In the first tutorial we learned how to see light and shadow, and in the second one, the principles of color. Today we're going to learn some advanced tricks that will give your artwork a real spark. The key word here is variety, in color and form. If sometimes the things you draw look like plastic figurines, this tutorial should help you a lot!

Materials

Most of the problems with painting colors lies in the depiction of surfaces. The surface structure influences our perception of color and brightness, and there are a lot of issues you need to take control of. When ignored, it can make for a dull, "plastic" scene. Plastic is the default material of every beginner's drawings—let's move past that.

Specular and Diffuse Reflection

In the previous tutorial I mentioned glossiness, but I didn't emphasize how important it is. In general, there are two kinds of color-making reflections: diffuse, and specular. Usually they're mixed together, and the proportion between them creates the overall reflection we perceive. So we can see matte, gloss, matte shine and all the stages in between.

As we noticed before, specular reflection is made by a ray reflected perfectly by the surface straight to our eyes. The more specular the surface, the clearer image of the light source appears on it. The less specular it is, the fuzzier the image, until it eventually becomes just a blurry spot of a diffuse reflection shifted to the light source's color. The shiny layer may be a property of the material, or just an effect added by water.

It's safe to treat every material as partially specular. Even a rubber ball or a plush has a little bit of shine. Using various specularity levels for the materials on your scene is very important for diversity. Shine is so powerful that it's tempting to use it everywhere, but flooding all the scene with oil isn't really the way to an attractive artwork.

A clear specular reflection doesn't always have the color of the light source. It works like this only if the "specular layer" (that's a simplification, but there's no need to go into technical details) reflects all the colors. If it doesn't, we get a red ball with clear green opalescence. It's a nice effect for gemstones, expensive fabrics, feathers (for example, ravens are black with blue tint) and the carapaces of beetles (blue tint on a green body).

The level of specularity should also be used to show how rough or polished the material is. A rough material after careful polishing will reflect a lot of light, so you should use a different specularity for an old wooden table and a polished wooden bowl—although they are made of the same material, their tooling made the difference.

Texture isn't only about shape of a surface and the way it should be drawn, but also about the reflective properties of the material. Every surface is made of tiny objects, and they all react to the light source too—they cast their tiny shadows and have their little highlights. That's why simply pasting a half-transparent textured photo over the top of a drawn material doesn't always make it look "right". The finer the texture, the less this effect occurs, but you need to be careful with bigger ones, like scales or bark. Also, every rough texture drastically decreases overall specularity of the surface!

A rugged or porous surface (like skin) disturbs the specular reflection

Fresnel Effect

The perceived specularity of a surface depends on the angle of view. The sharper the angle, the clearer the reflection. This effect is very helpful in finding a perfect place to define glossiness of our material, and it also tells us when to treat water or glass as a transparent material, and when it should work like perfect mirror. You can observe this phenomenon on wet floor—the lower you keep your head, the clearer the reflection.

Transparency and Refraction

Transparency is troublesome, because its intuitive definition is almost impossible to be conveyed into drawing. A simple change in the opacity of an object makes it look like a ghost, not like a glass. That's because our casual definition of "transparency" simplifies the issue.

Let's see how it works. Colored glass is easy to explain: for example, red glass absorbs all the colors, and only red passes through. Putting it simply, it works like a color filter.

Intuitively, a fully transparent material lets all the rays through, without any absorption or reflection. But if the rays didn't interact with the material in any way, how would we be able to see the material?

If you read the previous paragraphs carefully, you should guess the answer—only a 100% matte material reflects nothing. So even our pure glass reflects a bit of specular reflection, showing us the surface.

An interesting fact: specular reflection is what turns a lake of transparent water into a mirror!

But how do transparent objects like water drops or glass cast shadow? This is based on refraction, the bending of the rays when they pass between two media. Including this phenomenon in your painting gives a transparent object a sense of volume—this is the difference between a solid glass ball and a bubble.

You should remember this scheme from physics classes. The only thing we need to remember here is that the thicker the material, the more likely the rays will be disturbed.

The situation gets even more interesting when the surface is bent, creating a lens. Lenses have an amazing ability to focus or scatter rays. And when rays are focused (bent from their initial direction to one single point), areas of shadow appear. That's how a transparent lens creates a shadow!

Every transparent object with bent surface makes a lens. Every convex lens is able to focus light to some extent. Therefore, a wine glass, a bottle of water or a drop will all cast a shadow and very bright spot (or smudge, depending on how good the lens is) of focused light. If, additionally, the lens is colored, the bright spot will be colored too.

But what does such a convex lens do? Of course, it magnifies! That's the most important thing you need to paint realistic transparent, solid materials.

A bubble and a solid glass ball

Translucency and Subsurface Scattering (SSS)

What if material doesn't let the light completely through? What if it gets blocked somewhere along the way? Well, then we get a situation like below. The leaf is backlit, but it looks as bright as if the light source was right in front of it.

The mechanism behind this is very simple. Some materials are translucent—they're not fully opaque, nor transparent. Light that seems to be absorbed inside them sometimes finds its way out, but before this it gets scattered, creating an illusion of internal illumination. Of course, the longer the distance our light needs to travel, the weaker the transmission.

The most popular semi-translucent material is human skin. Subsurface scattering is the most visible in soft parts of our body, like ears or nose, but to some extent it can be observed everywhere. If you ignore this phenomenon, your painted face will look like a statue. Also, grass owes SSS its juicy green, as it comes mainly from transmission, not reflection.

What does subsurface scattering do to color? The most conspicuous sign of SSS is highly increased saturation in a place that isn't directly illuminated. Brightness can be higher too, and the temperature of hue shifts to the color of light.

The mechanism of SSS on a human skin is a big topic, but I've got something extremely helpful for you—an interactive human head model where you can adjust all the options yourself and see how it works!

Ambient Occlusion

Multiple light sources, or just one light source naturally scattered in the scene don't leave too much space for edge-defining shadows. Therefore, there's a high risk of flattening the image or making it too "soft". To avoid it you can focus on the absolute shadows—those which will be there no matter how many lights you use.

Using ambient occlusion doesn't mean your objects need to look like 3D models (although it is possible)—just define all the crevices. A good method for this is to imagine your object has been flooded with a non-stick dye. Most of it will run down, but some will be stuck in the crevices. The more dye there stays, the deeper the shadow.

An example of an object illuminated by directional light and with ambient occlusion

Fluorescence

Some materials are able to transform invisible light hitting them into visible light. These objects seem to reflect more light that there is in the environment—they look like they're glowing, but they don't emit light on their own. This effect can be useful for magic things, plants, fungi and mysterious animals.

Emission of Light

Sometimes we'll want to create the light source of some object of the scene. With all the things we've learned it should be a piece of cake! The brightness of our light source will define its power, and the hue and saturation are up to you—it's the source, it can be anything. Important: an illuminating object doesn't cast shadow!

Scene Coherence

I painted the picture below about two years ago. As you can see, the composition is great, the anatomy is decent, the colors are eye-catching, but... it doesn't look like a whole. Every object (dragons, reindeer, Santa) has their own set of colors and there's no relation between them. And how can there be no relation, if they're in the same scene, under the same light?

Colorful Light

Colorful light seems to be some additional, abstract topic. Sun gives us white light and it looks natural to us, and any colorful light needs to come from an artificial, man-made source. However, in the previous tutorial we noticed that sunlight is never perfectly white and neutral—it's either warm or cold. That leads directly to conclusion that it's white light that's artificial!

The hue of the light source affects all the objects in the scene, creating a coherent set. Look at these two pictures below. I'm sure you can easily tell which one is warm and which cold. They both look OK—the first one seems to be taken on a sunny day, and the other when it's more cloudy. What's important, is that if they weren't placed next to each other, you probably wouldn't even notice this yellow or blue tint! As we said before, color temperature comes from comparison.

You may be familiar with the concept of white balance. Sometimes photos look too yellow, or too blue. This is because a camera takes a picture of what it "sees", and we don't only see, we have a brain that changes the image immediately and imperceptibly. A photographer needs to shift the colors to yellow (if the photo is too bluish) or to blue (if it's too yellow), but our brain makes this correction outside of our awareness.

What does this mean? You can tell when something is white even if it doesn't look white under some circumstances. Check it out in the evening, when everything is blue. You'll know a sheet of paper is white, even though it's not—it doesn't reflect full spectrum to you! It's called chromatic adaptation and it leads to various illusions, like the one below.

The circles on the right look red, blue and green to us. Our brain assumes the background would be white under perfect light conditions (like in the lefthand picture) and calculates the difference between the white and the actual background. Then it adds this value to the other colors.

Of course, it's only an illusion. The colors are purple, blue and cyan, and these are the ones you should use in painting something lit by this kind of light source. In digital painting you can use a blue filter to shift all the colors to their proper hue, but it limits you to only one light source. How should you foresee how the colors will change under a colorful light? First we need to understand how it works.

These are the same colors as in the righthand image above. Check in your graphic editor if you don't believe me.

Just for revision—when white light (made of all the colors) hits a red object, all colors except red are absorbed. Red itself is reflected to our eyes. A white object reflects all the colors. Hopefully you've got a good grip on that now, because we're going to take it one step further.

What happens if we remove all the colors from the light, leaving only red? Our red object will be the same red as before, but the white object, still reflecting everything, will reflect only red too! In fact, now both these objects are indistinguishable.

Let's leave only blue light now. This time the red object absorbs it all and nothing gets reflected—it appears black. The white object again takes on the color of the light.

But those were extreme examples—in nature it's rarely this dramatic. Usually colorful lights are made of all the components, and the objects aren't colored strictly in red, green or blue only. Let's create a more realistic situation and see what happens.

The object in the images below is dark green and slightly glossy, like grass. It's not pure green, but rather, a combination of green, red and blue. You can see some parts of the light rays in the first image were absorbed (this is what a plant uses for photosynthesis), but they each one was partially reflected. Now, if we change the light source to orange (a lot of red and a bit less green) the final image in our head lacks blue and a bit of green. It's still a green ball, but you can see the difference. This difference is crucial for the object to fit the scene under each particular light source.

You can easily replicate any of these experiments using your monitor as a color lamp—just wait until it gets dark, then open your graphic editor in full screen mode and fill the whole page with the color you need. But you don't need to do it every time you want to paint something that's under a colored light. To check how a color will change under a certain light, you need to answer these questions about the scene:

  • What does the object need for its color to be shown?
  • What and how much color does it get from the light source?

When we've got two colors—of the object and of the light—we need to find an intersection between them. A yellow object and a magenta light have the same intersection as a magenta object and a yellow light. Therefore, we're looking for an intersection of two colors, no matter which is which. To simulate this behavior digitally, you can use the Multiply blending mode.

However, there's a funny thing about this mixing. Do you remember subtractive mixing, from the previous tutorial? This is just the same! It means you can use the four rules of subtractive mixing to foresee how an object will look under some light. Traditional artists shouldn't have any problems with this, but as a digital artist you just need to keep in mind a few other rules.

By reducing saturation you reduce the amount of "paint" and the presence of that color in the mixture. The brightness of the mixture is affected mostly by the luminance of the darker components. A small amount of paint means here your object is getting mostly white light, shifted slightly to another color.


Brightness can be seen as the amount of ambient light in the environment. For example, in the night the light source should be dark blue, and in effect the objects should be dark too. In movies night is often simulated by strong blue light and the scene is actually bright!

The brightness of the mix drops quite drastically most of the time, and although it is realistic, realism isn't always welcome. For example, real night scenes are very, very dark, not romantic blue made by long exposure on your camera and a few edits in Photoshop. But we actually want to see this romantic blue instead of fuzzy shapes in the dark. So using perfectly calculated values of intersection isn't always the best way for a nice effect—like many things in art, your own way of estimating the final color forms a part of your style.

From left to right: daylight (template colors), night (template colors with simple blue filter), night (colors shifted)

Color Mixing: Reflected Light

The process of reflecting light within the scene is crucial for making it a whole rather than a set of objects. It's a common mistake to shade every object totally on their own, which results in a fake-looking scene. Reflected light is nothing but a colorful light coming from a different direction than the direct light. Therefore, it obeys the rules we've just talked about.

However, there's one more rule you need to remember. Brighter light always beat weaker light. It means that reflected light will never be stronger than direct light (it may be just exactly as strong, if it's perfectly specular reflection), and it will be observed in the shadow only. And if the reflected light is darker than the core shadow, it won't be visible too—there is no such thing as black light, only the lack of it. If you see a "reflection" of a dark object on a bright, specular surface, it's actually a kind of shadow—you see bright areas outlining the lack of reflection.

But reflection isn't only about what color is bounced between two surfaces. It's also about how and when it is bounced. So here come a bunch of simple rules that you need to keep in mind:

  1. Fully matte surfaces don't reflect any light to each other;
  2. Fully specular surfaces reflect everything—they work like a light source with sharp edges. They are able to make the surface as bright as if it was illuminated directly by the light source;
  3. Mixed surfaces (slightly glossy or specular coated with matte) reflect as much light as there is specularity in them—the glossier, the stronger reflection;
  4. Dark surfaces are dark because they absorb a lot of light. Therefore, they don't influence matte objects. On glossy surfaces there may appear a shadow of them.
  5. White surfaces reflect everything—they brighten objects a lot.

Multiple Light Sources

Let's talk some more about light sources. Since they create the color, they're crucial to painting a scene as we imagine it. We can distinguish many kinds of light sources, with the most characteristic being:

  1. Sunlight: strong, but kind of diffuse light source. Its shadows can be soft or sharp depending on its power (e.g.: sun, fire, lamp);
  2. Spotlight: strong, directional light with sharp shadows (e.g. flashlight, sunlight passing through a hole);
  3. Reflected light: light bounced from an object to another;
  4. Ambient light: diffuse reflected light without a defined direction (e.g. sunlight reflected from the sky, light of fireplace reflected from the walls);
  5. Transmitted light: light passing through a translucent object.

Mixing these light sources can help create a nice, appealing scene. Beginners often start with side sunlight only, since it's the most obvious. However, it creates dull shadows covering a big part of the scene. Ambient light helps fill these shadows to reveal the shapes hidden under them. On another level artists link all the objects on the scene by including reflected light. Sometimes transmitted light comes to play too. How to manage all this chaos?

Even if your scene seems to be flat, you'll need to use perspective for its lighting. That's a rule you can't break—shadows and highlights appear on forms, not on a flat sheet of paper. Side lighting is used the most because it's easy to place the light source mentally on the left or on the right of a 2D surface, but it's also overused and boring. If you want to take full control over the lighting in your scene, you need to plan it.

Let's say you want to draw a composition like this. It doesn't need to be that simple, but every scene can (and should) be simplified to primary forms in your mind.

Now you can change your perspective. Take a sheet of paper and sketch your scene from above and/or in front view. This way you'll be able to place the light sources at any angle you wish. You'll also see how the objects interact and where the shadows are casted or overlapped. This sketch should be very, very simple - the details of objects will inherit their shading. You'll probably cope without it in simple scenes, but when things get complicated (uncommon light sources, a lot of transmission) it's indispensable.

Shadows

When dealing with multiple light sources, including colorful ones, you may stumble upon a new problem—what happens to the shadows?

Ambient light is known by its ability to color the shadows made by the primary light source. It'll never create new shadows in the light area, but it's able to cast its own shadows in the shadow area.

Reflected light is sometimes cast into shadow, coloring and brightening it.

Transmitted light is good at destroying the shadow of its object. Sometimes, when it's weak, it only colors the shadow as an ambient light.

The rule of thumb has it that shadow should be a complementary color to the light. For example, blue light casts yellow shadows and vice versa. It's true only to some extent—we need two light sources to make it happen, and it's useful only if one of them is primary or secondary color, and the other one is white.

This optical illusion is based on a very interesting vision mechanism called color opponency. Cones aren't the only middleman between what we look at and our brain. Surprisingly, the three signals aren't transferred directly—they go through three channels: red/green channel, yellow/blue channel and light/dark channel. That's why there's no bluish yellow—only one of these colors can go through a single channel at a time.

The main conclusion we need to take from this is that our brain doesn't see red because it receives red signal, but only because it doesn't receive green nor blue at the same time. When you see a yellowish (RG) shadow casted by blue (B) light source in the presence of white (RGB) light source, it's because the shadow (RGB) is a bit less blue than the light area (RGB+B). And for our brain, if it's not blue, it's yellow! Analogously, when something isn't bright, it's dark. It's the same with the afterimage phenomenon—a white screen becomes less red after long time of looking at something more red, and thus it gets a complementary tint (cyan).

Look at the dot on the left by 15 seconds, then look at the dot on the right

Of course, it's a very subtle coloring and if you ignore it, no one should notice a thing. I described it only because I hear this rule quite often, and people seem to use it without trying to understand it. Most of the circumstances that seem to confirm this rule are a result of ambient light coloring (yellow sunlight—blue shadow made by sky; orange lantern light—dark blue sky in the night), so when you see an oddly colored shadow, check the ambient light first.

And what about another rule of thumb: "warm light—cold shadows; cold light—warm shadows"? Well, yes, it's kind of correct, but only if you expand it: warm primary light— cold ambient light and vice versa. This kind of contrast is very pleasant to the eye, but it's not some kind of rule that must be obeyed everywhere and at every time. It's you who chooses the color of ambient light. And most certainly you shouldn't try to add cold shadows when your ambient light is actually warm!

Conclusion

So, that would be all the fundamentals you need to know about color. Of course, it was just a concentrated summary and you're encouraged to study all these topics further. If you think there's too much to learn, just remember—painting isn't easy! It may look so when a professional creates a masterpiece in a matter of minutes, but it's only because they have spent years polishing their skills. Painting isn't just about putting colors on paper or screen, it's about knowing how to put them, knowing where they come from, where they should and shouldn't be, and what they should look like. If you really want to be good, don't rely on your talent or feeling only. Take the time to master theory, all this boring stuff hidden behind art. You'll be surprised how many of your unvoiced questions it can answer!

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